1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an optical pickup device suitable for a compatible optical pickup device for emitting several kinds of laser beams having different wavelengths to a recording medium.
2. Description of the Related Art
Currently, various optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have been commercialized and widely used. Further, recently, next-generation DVD standardization for recording and reproducing information using a blue-violet laser beam has been proceeded with. In the next-generation DVD, information is recorded and reproduced using the blue-violet laser beam having a wavelength of about 405 nm. When the wavelength of the laser beam shortens, a higher density can be obtained.
Therefore, when the variety of optical discs increases, development of a so-called compatible optical pickup device capable of performing recording and reproduction on different kinds of optical discs is desired. In order to irradiate an optical disc with laser beams having different wavelengths, it is possible to employ an arrangement in which semiconductor lasers that emit laser beams having different wavelengths are separately disposed. However, when such arrangement is employed, spaces for separately disposing the semiconductor lasers and optical elements for guiding the laser beams to an objective lens are required. Consequently, the external dimensions become large and the number of parts increases.
Thus, an arrangement in which a plurality of laser elements having different emitting wavelengths are provided all together in a single CAN package has been studied. According to such arrangement, a space for disposing the semiconductor lasers can be reduced and an optical system can be commonly used among the laser beams.
However, when the plurality of laser elements are provided in the single CAN package as described above, a deviation in a direction perpendicular to the optical axis occurs between the optical axes of the laser beams according to arrangement gap between the respective laser elements. In this case, when the optical axis of the optical system is aligned with the optical axis of a laser beam, the optical axes of other laser beams deviate from the optical axis of the optical system. Consequently, in the case of recording and reproduction using the other laser beams, there arises a problem in that aberration of laser beams is produced on a recording medium to cause deterioration of optical characteristics.
Therefore, according to JP 06-131688 A, a birefringence element is disposed immediately after a semiconductor laser including several kinds of laser elements, and the optical axes of the laser beams are aligned with one another by the birefringence element to guide the laser beams to the optical system.
However, this conventional art requires an additional birefringence element. In addition, it is necessary to form the laser elements such that the polarization plane of the reference laser beam is orthogonal to the polarization plane of each of the other laser beams. However, it is hard to form laser elements in which polarization planes of laser beams are made different from one another. Also, since the birefringence element is expensive, a problem occurs in that a cost of the optical pickup device as a whole increases.
To solve these problems, applicant has filed JP 2004-145169 A. In this application, a technique for optical axis correction using a diffraction grating is proposed. According to this technique, it is possible to suppress an increase in cost of optical pickup device because of using a diffraction grating. Also, it is not need to adjust the process to form laser elements while considering the state of the polarization plane of each laser beams having different kinds of wavelength.
However, when the optical axis correction is performed by using the diffraction grating as described above, there arises a problem in that power of laser beams decrease because of diffraction efficiency of the diffraction grating with respect to for each wavelength.
For example, in case that the optical pickup device is a compatible device for CD/DVD/next-generation-DVD, when the optical axes of the infrared laser beam (for CD) and the red laser beam (for DVD) are aligned with the optical axis of the blue-violet laser beam (for next generation-DVD) by using the diffraction grating, not only the power of the infrared laser beam but also the power of the red laser beam decrease in the optical axis correction for the infrared laser beam, still, not only the power of the red laser beam but also the power of the infrared laser beam decrease in the optical axis correction for the red laser beam. Thus, the total decrease power of these laser beams in the optical axis correction for these laser beams get to quite large amount.